(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and, more particularly, to a lateral-electric-field-mode LCD device such as an in-plane-switching-mode (IPS-mode) LCD device.
(b) Description of the Related Art
LCD devices are categorized in two types: a first type wherein orientation of a liquid crystal (LC) layer in the LCD device is rotated in a plane normal to the surface (substrate surface) of the substrates sandwiching therebetween the LC layer; and a second type wherein the orientation of the LC layer is rotated in a plane parallel to the substrate surface. The first type includes a twisted-nematic-mode (TN-mode) LCD device as a typical example, whereas the second type includes an IPS-mode LCD device as a typical example.
In the TN-mode LCD device, since the LC layer has an orientation deviated from a plane parallel to the substrate surface, a larger viewing angle may cause a larger deviation in the polarization angle of the transmitted light. As result, it narrows down the viewing angle. On the other hand, in the IPS-mode LCD device, since the LC layer has an orientation in a plane parallel to the substrate surface, a higher viewing angle does not cause a deviation in the polarization direction, thereby achieving a wider viewing angle characteristic. This leads to a recent tendency for employment of a larger number of lateral-electric-field-mode (LEF-mode) LCD devices. However, although the LEF-mode LCD device thus achieves a wider viewing angle characteristic in comparison with the TN-mode LCD device, it is known that the LEF-mode LCD device involves a problem of coloring or color deviation in a larger degree as observed in a slanted viewing direction.
In general, LCD devices are designed to achieve an almost uniform optical intensity for the RGB primary-color components of the light transmitted by the LC layer in the direction normal to the substrate surface. However, if the angle of the incident light (incident angle) is deviated from the direction normal to the substrate surface, retardation for the LC layer changes and thus the optical intensity of the transmitted light changes.
The retardation is determined by a product of the optical path length by the refractive index anisotropy, and does not depend on the wavelength of the transmitted light. However, the optical intensity of the RGB components of the transmitted light changes independently of each other, because the optical intensity of the transmitted light changes depending on the ratio of the retardation to the wavelength of the transmitted light, thereby causing a significant range of variation in the optical intensity among the RGB components of the transmitted light. Thus, the image observed in the slanted viewing direction involves a coloring, wherein one or two of the RGB components of the transmitted light are intensified to change the original color image. The coloring thus degrades the image quality of the original picture.
Patent Publication JP-3120751B describes a LCD device wherein each pixel includes two sub-pixel areas having different directions of the electric field applied to the LC layer, More specifically, the orientation of the LC layer is rotated by the different directions of the electric field in is the opposite rotational directions between the sub-pixel areas while keeping the symmetric relationship in the orientations between the sub-pixel areas. It is recited in the publication that this configuration compensates the optical characteristics of both the sub-pixel areas therebetween to suppress the coloring.
In the configuration of the above publication, the coloring is suppressed only when the gray-scale level represents a bright state, wherein the orientations of the LC layer in both the sub-pixel areas are perpendicular to one another. If the gray-scale level represents a dark state or an intermediate state wherein the angle in the orientations of the LC layer between both the sub-pixel areas is deviated from the 90 degrees, the compensation of the optical characteristics is insufficient, whereby the coloring is not suppressed in a desired degree.
FIG. 8 shows the LCD device described in Patent Publication JP-2828073B, which describes a technique for providing different thicknesses for the color filters of the RGB pixels in the LCD device. In FIG. 8, RGB pixels 101R, 101G and 101B include color filters 23R, 23G and 23B having different thicknesses d″R, d″G and d″B, whereby the LC layer 30 has different thicknesses dR, dG and dB in the RGB pixels 101R, 101G and 101B, respectively. The ratio dR:dG:dB in the LC layer 30 is set equal to the ratio λR:λG: λB, wherein given λR, λG and λB are the representative wavelengths of the color filters for the respective RGB pixels. The representative wavelengths are each selected from within the wavelength range having 70% of the peak wavelength in the spectrum of the light passed by the respective color filters 23R, 23G and 23B of the RGB pixels.
In the LCD device described in JP-2828073B and having different thicknesses of the RGB color filters, the configuration wherein dR:dG:dB=λR:λG:λB allows the ratio of the retardation to the wavelength to have a substantially same value among the RGB pixels, independently of the incident angle with respect to the substrate surface. This suppresses the range of variation in the optical intensity among the RGB components of the transmitted light, to thereby suppress the coloring in the image as observed in a slanted viewing direction.
In the LCD device described in JP-2828073B, it should be noted that the color filters 23R, 23G and 23B having different thicknesses must have the same chromaticity for the RGB components of the transmitted light irrespective of the different thicknesses. Thus, the pigments mixed in the respective color filters must be adjusted in consideration of the thicknesses of the respective color filters to achieve the same chromaticity for the RGB components.
This adjustment of the pigments necessitates a variety of design works including calculation of the absorbed spectrum, determination of the concentration of the pigments, viscosity of the pigment paste, measurement of the chromaticity for the RGB components achieved, and correction of the chromaticity based on the measurement. That is, the adjustment of the pigment necessitates complicated design work and may raise the costs of the LCD device.